Cellular and molecular mechanisms of repair & regeneration

Define the difference between repair and regeneration in the nervous system

• Repair: simpler, restores partial structure or function; may not fully recreate original anatomy

• Regeneration: complex, restores original anatomy, physiology, and ideally function of axons

What is axonal sprouting and how does it differ from regeneration?

• Sprouting: formation of new branches from existing axons

• Differs from regeneration: does not recreate the original axon, may restore some function but not the original anatomical pathway

How is regeneration assessed anatomically, physiologically, and behaviourally?

• Anatomy: histology, immunostaining, markers to see axon growth

• Physiology: tracing (anterograde/retrograde) to confirm signal conduction

• Behaviour: functional recovery tests; may occur even without full regeneration

Explain anterograde and retrograde tracing techniques

• Anterograde: tracer injected in cell body → flows to axon terminals

• Retrograde: tracer injected in axon terminal → flows back to cell body

• Used to determine axon continuity and regeneration

Why might functional recovery occur even without true regeneration?

Due to sprouting and plasticity, which can restore network connectivity even if the original axon does not fully regenerate

Compare CNS and PNS environments in terms of regeneration permissiveness

• PNS: permissive environment; supports axon growth and regeneration

• CNS: inhibitory environment; glial scars, myelin inhibitors, and ECM molecules limit regeneration

What role do CSPGs play in CNS injury?

Chondroitin sulfate proteoglycans (CSPGs) in glial scars inhibit axonal regeneration by activating inhibitory receptors on neurons

Name key myelin-associated inhibitors that block CNS regeneration

Nogo, MAG (myelin-associated glycoprotein), OMgp (oligodendrocyte-myelin glycoprotein)

Explain the Rho/ROCK pathway’s role in axon growth inhibition

Activated by inhibitory signals (myelin, CSPGs) → reorganises cytoskeleton → collapses growth cone → blocks axon elongation

How does cytoskeleton dynamics contribute to axonal regeneration?

• Actin and microtubules reorganise to form growth cones

• Myosin motor proteins generate forces for axon extension

• Proper dynamics are essential for regeneration

What are regeneration-associated genes (RAGs) and where are they upregulated?

• Genes that promote axon growth and repair

• Strongly upregulated in PNS neurons after injury; weakly expressed in CNS neurons unless primed

Describe the “conditional injury” experiment and its significance

• Injury to peripheral branch of DRG neuron “primes” CNS branch for regeneration

• Shows that intrinsic cellular programs can overcome inhibitory CNS environment

How do PTEN and SOCS3 influence axonal regeneration?

• PTEN: inhibits mTOR pathway → suppresses growth

• SOCS3: negative feedback on growth factor signalling

• Knocking down both enhances CNS axon regeneration synergistically

Why is combining intrinsic and extrinsic factor manipulation more effective than targeting one factor?

• Extrinsic inhibition (environment) limits growth

• Intrinsic programs (gene expression, metabolism) drive growth

• Combining approaches overcomes both barriers for stronger regeneration

What early cellular events occur immediately after axonal injury?

Membrane disruption, calcium influx, cytoskeletal rearrangement, growth cone formation, and activation of pre-existing proteins

How does calcium influx influence regeneration?

• Activates pre-existing proteins

• Triggers cytoskeletal reorganisation and growth cone assembly

• Initiates early signalling before gene transcription occurs

Describe the role of transcription factors and epigenetic regulation in regeneration

• Transcription factors (STAT3, ATF3, etc.) bind promoters of RAGs to initiate gene expression

• Epigenetic regulation (DNA methylation, histone acetylation) controls promoter accessibility

• Open chromatin is required for transcription factors to activate regeneration genes

Explain why chromatin accessibility is different in CNS versus PNS neurons

• CNS: promoters often closed/heterochromatic → transcription factors cannot access RAGs → limited regeneration

• PNS: promoters open → transcription factors activate RAGs → robust regeneration

What are the limitations of optic nerve regeneration experiments?

• Axons may regrow without myelin restoration → no functional recovery (e.g. vision not restored)

• Regeneration may require multiple intrinsic and extrinsic manipulations

Why does the CNS prioritise stability over regenerative capacity from an evolutionary perspective?

• CNS evolved for long-term stability and precise connectivity

• Excessive regeneration could disrupt established circuits, impairing critical functions